Scanning probe microscopes (SPMs) are instruments that provide high resolution information about surface contours. Vertical movement of a sensing probe, in response to a raster scanning procedure of the sensing probe across a target surface, is used for determining the target surface contour. Examples of SPM devices include implementations based on the interaction of attractive forces including atomic, electrical or magnetic to maintain a constant probe to target surface gap, or distance. One common use of these devices is imaging. Some types of SPMs have the capability of imaging individual atoms.
In addition to imaging surface contours, SPMs can be used to measure a variety of physical or chemical properties with detail over the range from a few Angstroms to hundreds of microns. For these applications, SPMs can provide lateral and vertical resolution that is not obtainable from any other type of device. Examples of applications include imaging or measuring the contour properties of transistors, silicon chips, disk surface, crystals, cells, or the like.
In order to provide for high resolution information about surface contours, variables for the SPM include the effective size of the scanning probe, the positioning of the scanning probe above the target surface, and the precision of the scanning device itself. A precise scanning probe measurement can take tens of minutes to complete. During the measurement period, any movement of the sample relative to the probe degrades the accuracy of the data, for which compensation or correction may not be available. The resulting measurement is therefore less precise than a measurement taken without relative movement. The major components of the relative movement between the sample and the probe are mechanical vibration of the scanning probe microscope body itself and thermal creep of the scanning probe microscope components within a thermal path between the scanning probe assembly and the sample.
Mechanical vibration is a practical consequence effecting the accuracy of any precise measurement. As the relative magnitude of a measurement approaches the nanometer to Angstrom range, as in scanning probe microscopes, the component effect of vibration increases as an absolute magnitude of the overall measurement.
Thermal creep is also present in precise measuring devices. In this context, thermal creep refers to the relative motion of the sample versus the probe tip caused by a change in temperature of the scanning probe microscope components in the thermal path between the scanning probe assembly and sample. As a time dependent function, thermal creep need not be linear nor monotonic, and accordingly compensation or correction may not be fully afforded. Thermal creep is a function of many parameters including total path length of structural materials that hold the sample in position, thermal expansion coefficients of these materials, magnitude and application of thermal gradients, and thermal mass of materials.
The elements of mechanical vibration and thermal creep both vertically and horizontally affect scanning probe positioning relative to the sample. In standard scanning probe applications of small target areas, the resolution in the vertical axis is an order of magnitude greater than the resolution in the horizontal axis. Thus, vertical compensation for mechanical vibration and thermal creep is, at a minimum, required in standard applications.
Large samples require supporting structures large enough to provide a range of motion great enough to scan the entire sample surface. With increasing dimensions of supporting structures, the effect of horizontal vibration is more significant. Thus, as the target area of the surface to be sampled approaches that of production size samples, the need for horizontal compensation of mechanical vibration and thermal creep increases. The need to provide horizontal compensation of mechanical vibration and thermal creep is particularly important where the objective of a scanning probe system is to make precision horizontal measurements. This is particularly important in critical dimension (CD) metrology.
The need to improve accuracy of data in sensitive apparatus, such as scanning probe microscopes or scanning tunneling microscopes, has been addressed by vibration damping or isolation. U.S. Pat. No. 4,908,519 to Park et al an U.S. Patent to Bednorz et al illustrate, for example, spring mass damper vibration isolation systems. A shortcoming of these systems are that only small samples may be scanned and the systems offer no compensation for thermal creep. U.S. Pat. No. 4,947,042 to Nishioka illustrates a flux channeling bar magnet to pull a scanning head onto a sample mount. Although rigidity of the structure is enhanced, the embodiment does not address thermal creep.
In view of the fact that the resolution of the new microscope developments and the requirements in electronic circuit manufacturing have increased over several orders of magnitude, it has become necessary to design new sample holding device which avoid the disadvantages of the prior art.